This invention relates generally to fiber optic communications and, more specifically, to signal processing optical transmitters for use in fiber optic communication systems.
Conventional communication systems, such as cable television systems and other broadband network systems, are designed for the distribution of video signals from a main transmitting station (commonly referred to as the xe2x80x9chead endxe2x80x9d) to subscribers in a local service area. At the head end station, video information is received from satellite transmitters and demodulated for retransmission through fiber optic cables to a number of nodes. Hardware assemblies within each node are designed to receive the optical signal transmitted by the head end station and retransmit the information as a radio frequency (RF) signal to individual cable subscribers. A tree-and-branch architecture, complete with necessary RF amplifiers, is used to distribute the signals from the nodes to each subscriber.
While this architecture is adequate for the distribution of conventional television video signals to a large number of users, there is currently a desire to provide additional services that may include telephony, high definition digital television, and two-way digital service, which could include Internet connectivity. In order to address these requirements, the portion of the RF spectrum below 750 MHz can be subdivided into three bands, each fulfilling a different communication function. For example, the region from 50-550 MHz could carry conventional analog television signals. Assuming a 6 MHz channel spacing, eighty analog television channels could be transmitted within this frequency window. In addition, digital channels could occupy the 550-750 MHz band, and the 5-40 MHz region could be allocated for reverse channels, i.e., could be allocated for the transmission of return signal upstream from the subscriber to the head end.
While the transmission architectures needed for distributing analog and/or digital information from a central station to many subscribers is well known, there are a number of technical problems that much be addressed before the reverse transmission channel is feasible. It is known that analog links in the 5-40 MHz window that are designed to connect many subscribers to a single node are vulnerable to RF noise problems. Since noise signals from the many subscriber locations are summed at the RF/optical node, it is possible to saturate the receiver with the accumulated noise. In the most extreme case, it is possible for a single, high noise subscriber to drown out the signals from others connected to the same node. Unfortunately, there are many common noise sources in the 5-40 MHz band. These include CB radios, motor noise, etc. Filtering out these sources and/or designing an analog reverse transmission system that is immune to them is difficult and expensive.
U.S. patent application Ser. No. 09/102,344 to Farhan et al., entitled xe2x80x9cDigital Optical Transmitterxe2x80x9d and assigned to the assignee hereof, describes a reverse transmission architecture to address these problems, and the teachings of Farhan are hereby incorporated by reference. According to Farhan, information from each subscriber location is transmitted from the subscriber to the RF/optical node using an RF modulation format that has a high level of noise immunity. Quadrature phase shift keying (QPSK) is one example of such a format that offers significant improvements with respect to conventional analog formats. Specifically, the signal-to-noise ratio required for QPSK transmission is roughly half that required for analog modulation.
At the node, the reverse signals from many subscribers are summed in an RF combiner and converted to a digital format using a conventional analog-to-digital (A/D) converter. According to Farhan, this signal may be combined with a digital pilot tone, converted from a parallel to a serial bit stream, and subsequently used to modulate a laser diode. The output of this diode is transmitted by fiber optic cable to the head end receiver. Fiber optic amplifiers may optionally be placed between the node and the head end to overcome optical fiber losses.
In Farhan, the basic noise issues associated with subscriber-to head end transmission are addressed, but hardware analog filters must be built into the RF receiver at the node when it is initially installed. Changes in the number of subscribers served by the node or changes in other features in the subscriber-to-node leg of the reverse link must be anticipated in the initial receiver design or achieved through hardware modification. These solutions can be difficult and expensive.
In current nodes, return signals may also be difficult to channelize. In many cases, several types of information with different bandwidth requirements are included in the return signal. For example, a typical home subscriber might generate set-top controller signals with low information content, voice telephony signals with medium amounts of information content, and digital modem signals with high information density. In an ideal case, the signal leaving the node would be a combination of different modulation formats, each selected to most efficiently handle a particular type of data signal. However, separation of these signals at the node and efficient retransmission to the head end cannot be performed using known communication system architectures. Additionally, data compression, which would be advantageous in situations in which multiple inputs are to be multiplexed onto a single output line, is difficult using prior art techniques.